What is a Döbereiner triad? Why can't the following three elements form such a triad? N (14); P (31); As (75) (Atomic masses are given in parenthesis)

When three elements with similar properties are arranged in order of their increasing atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements.

The atomic mass of phosphorus is not the average of the atomic masses of nitrogen and arsenic. / Average of atomic mass of N and As = \(\displaystyle \frac{14+75}{2}=44.5\) But atomic mass of phosphorus is $\displaystyle 31$ So N, P and As cannot be classified as Dobereiner's Triad